1. Technical Field of the Invention
The present invention relates generally to a stator which may be used with an electric rotating machine and designed to minimize leakage current, and an electric rotating machine equipped with such a stator.
2. Background Art
Japanese Patent First Publication No. 2009-112186, assigned to the same assignee as that of this application, discloses an electric rotating machine equipped with a hollow cylindrical stator in which a plurality of slots are formed. The slots extend in an axial direction of the stator. The stator is equipped with a stator winding made of wire. The stator winding has in-slot portions and turned portions. The in-slot portions are disposed inside the slots. Each of the turned portions connects between two of the in-slot portions outside the slots.
As viewed in a transverse section of the stator, the in-slot portions are arrayed, like in FIG. 17, in the form of multiple layers aligned in a radial direction of the stator. In FIG. 17, in-slot portions 320a are laid to overlap each other within a slot 330 in the radial direction of a stator 300. In the case where the stator winding is a three-phase winding, it is usually made up of a U-phase winding, a V-phase winding, and a W-phase winding. Ends of the U-, V-, and W-phase windings are disposed outside the slots in the radial direction of the stator because they must be connected electrically to a controller. Portions of the stator winding which are close to the ends of the U-, V-, and W-phase windings are, therefore, disposed in the slots as outermost layers.
The electric potential developed at the stator winding will be described below. FIG. 18 is a schematic view which shows a typical star-connected stator winding made up of U-, V-, and W-phase windings. Such a stator winding is wound in the stator with portions (i.e., the in-slot portions) aligned in the radial direction of the stator within the slots. In FIG. 18, “A” indicates a winding portion closer to an end U of the stator winding. “B” indicates a winding portion of the U-phase winding closer to a neutral point N. If a joint X of the winding portions A and B lies just at the middle between the end U and the neutral point N, an average potential developed at the joint X will be, as demonstrated in FIG. 19, a one-half (½) of that developed at the end U. Thus, the winding portion A closer to the end U is higher in potential than the winding portion B. The same applies to ends V and W.
In the case where the stator winding is made of a flat wire (also called a rectangular wire), side surfaces of the in-slot portions 320a, as illustrated in FIG. 20, face the stator core 310, so that capacitors are created between each of the in-slot portions 320a and an inner wall of the slot of the stator core 310. The in-slot portions 320a arrayed within the slot in the radius direction will also be referred to below as a first to fourth layers starting from the outmost in-slot portion 320a. Locations where the first to fourth layers are disposed in the slot will be referred to below as a first, a second, a third, and a fourth layer positions.
FIG. 20 shows transverse cross-sections of the first to fourth layers within the slot of the stator core 310.
An electrostatic capacitance C1, as established by the in-slot portion 320a in the first layer position, is given byC1=C10+C11+C12
Similarly, capacitances C2 to C4, as created by the in-slot portions 320a in the second to fourth layer positions, are given byC2=C21+C22C3=C31+C32C4=C41+C42
FIG. 20 illustrates the transverse cross-section of each of the in-slot portions 320a as being rectangular since the stator winding is made of a flat wire. Actually, short sides of each of the in-slot portions 320a is much smaller than long sides thereof.
Accordingly, the capacitance C10 in FIG. 20 is much greater than the capacitances C11 to C42. Specifically, C10>>C11, C12, C21, C22, C31, C32, C41, C42.
The following relation is, therefore, met.C1>>C2,C3,C4
If average potentials at the in-slot portions 320a placed in the first and fourth layer positions are defined as V1, V2, V3, and V4, a leakage current that is a time-derivative of a total charge is expressed byΣdQ/dt=Σd(Ck·Vk)/dt(k=1,2,3,4)
In the case where the in-slot portion 320a of the stator winding placed in the outermost layer position (i.e., the first layer position) is close to the end of any of the U-, V-, and W-phase windings, the average potential V1 in the outermost layer position, as described above, will be high, thus resulting in an increased leakage current. This usually leads to the problem of the so-called radio noise.